Transcript Slide 1

MECH3005 – Building Services
http://www.hku.hk/bse/mech3005/
Air Conditioning & Refrigeration:
Thermal Comfort
Dr. Sam C M Hui
Department of Mechanical Engineering
The University of Hong Kong
E-mail: [email protected]
Contents
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What is Thermal Comfort?
Thermal Environment and Heat Balance
Comfort Equation and Prediction
Influencing Factors
Environmental Indices
Local Thermal Discomfort
Thermal Comfort Measurements
Acknowledgement
• Cartoons and some figures are taken from:
• http://www.innova.dk/
* The need to define “comfortable environment”
arose from the AC industry
What is Thermal Comfort?
- That condition of mind
which expresses satisfaction
with the thermal environment.
ISO 7730
Thermal Environments
Thermal Comfort is a matter of many
parameters - Not only the air temperature.
Body Temperature
37 oC
• Normal body core temperature: 37 oC.
• We have separate Heat- and Coldsensors.
34 oC
• Heat sensor is located in hypothalamus.
Signals when temperature is higher than
37 oC.
• Cold sensors are located in the skin.
Send signals when skin temperature is
below 34 oC.
• Heating mechanism:
• Reduced blood flow.
• Shivering.
• Cooling mechanism:
Hot
Cold
• Increased blood flow.
• Sweating (Evaporation).
Perception of Thermal Environment
Warm
impulses
Cold
impulses
Activity
• Heat sensor in
Hypothalamus send impulses
when temperature exceeds
37 oC.
• Cold sensors sends impulses
when skin temperature
below 34 oC.
• The bigger temperature
difference, the more
impulses.
• If impulses are of same
magnitude, you feel
thermally neutral.
• If not, you feel cold or
warm.
The Energy Balance
Heat
Produced
Heat
Lost
• Thermal Comfort can only be maintained
when heat produced by metabolism equals
the heat lost from body.
Heat Balance Equation
• General heat balance
S = M - W - E - (R + C)
where
S = rate of heat storage of human body
M = metabolic rate
W = mechanical work done by human body
E = rate of total evaporation loss
R + C = dry heat exchange through radiation &
convection
Heat Balance Equation
• Rate of heat storage, S
• proportional to rate of change in mean body temp.
• normally, S is zero; adjusted by the thermoregulatory system of the body
• Metabolic rate, M
• heat released from human body per unit skin area
• depends on muscular activities, environment, body
sizes, etc.; unit is “met” (= 58.2 W/m2)
• 1 met = seated quiet person (100 W if body surface area
is 1.7 m2); see also the table in Figure 1
Heat Balance Equation
• Mechanical work, W
• energy in human body transformed into external
mechanical work
• Evaporative heat loss, E
• release of latent heat energy from evaporation of
body fluid
• respired vapour loss, Eres (respiration heat losses: latent
Erel and sensible Erec)
• evaporative heat loss from skin Esk (include skin
diffusion Edif and regulatory sweating Ersw)
Heat Balance Equation
• Dry heat exchange, R + C
• through convective and radiative heat transfer
• heat loss by radiation if skin temp. > temp. of
surrounding surfaces
• heat loss by convection if skin temp. > dry bulb temp.
• mean radiant temperature (tr) is that uniform
temp. of an imaginary black enclosure which
result in the same heat loss by radiation as the
actual enclosure
The Energy Balance
The dry heat loss
(R+C) represents
~70% at low Clovalues and ~60% at
higher Clo-values

Conduction (K) is
normally insignificant
compared to the total
heat exchange

• Parameters influencing the Heat Loss from a person
Conditions for Thermal Comfort
o
C.
• Two conditions must be fulfilled
to maintain Thermal Comfort:
34
33
32
31
30
29
• Heat produced must equal heat lost
• Signals from Heat- and Coldsensors must neutralise each other
0
1
2
Sweat prod.
W/m2
3
4
Metabolic Rate
100
80
60
40
20
0
1
2
3
4
Metabolic Rate
• The sweat production is used
instead of body core temperature,
as measure of the amount of
warm impulses.
• Relation between the parameters
found empirically in experiments.
• No difference between sex, age,
race or geographic origin.
The Comfort Equation
Predication of Thermal Comfort
• Fanger’s comfort criteria
• developed by Prof. P. O. Fanger (Denmark)
• Fanger’s comfort equation:
f (M, Icl, V, tr, tdb, Ps) = 0
where M = metabolic rate (met)
Icl = cloth index (clo)
V = air velocity (m/s)
tr = mean radiant temp. (oC)
tdb = dry-bulb temp. (oC)
Ps = water vapour pressure (kPa)
Predication of Thermal Comfort
• Fanger’s equation is complex
• but it may be transformed to comfort diagrams
• it can also be used to yield three indices:
• predicted mean vote (PMV)
• predicted percentage of dissatisfied (PPD)
• lowest possible percentage dissatisfied (LPPD)
Predication of Thermal Comfort
• PMV
• a complex function of six major comfort parameters;
• predict mean value of the subjective ratings of a group
of people in a given environment
• PPD
• determined from PMV as a quantitative measure of
thermal comfort
• ‘dissatisfied’ means not voting -1, +1 or 0 in PMV
• normally, PPD < 7.5% at any location and LPPD < 6%
Predicted Mean Vote scale
- +3 Hot
- +2 Warm
- +1 Slightly warm
- +0 Neutral
- - 1 Slightly cool
- -2 Cool
- -3 Cold
The PMV index is used to quantify the degree of
discomfort
Calculation of PMV index
PMV = (0,303e-2,100*M + 0,028)*[(M-W)- H - Ec - Cres - Eres]
PMV ?
PMV = (0,303e-2,100*M + 0,028)*[58,15*(M-W)
-3,05*10-3*[5733-406,7*(M-W)-pa]-24,21*[(M-W)-1]
-10-3*M*(5867-pa)-0,0814*M*(34-ta)
-3,96*10-8*fcl*[(tcl+273)4 - (teq+273) 4] - fcl*hc,eq*(tcl-teq)]
hc,eq = 2,38*(tcl - teq )0,25
M [MET)]
fcl
1,00+0,2*Icl for Icl <0,5 clo
1,05+0,1*Icl for Icl >0,5 clo
Icl [CLO]
PMV and PPD
• PMV-index (Predicted Mean Vote) predicts the subjective
ratings of the environment in a group of people.
• PPD-index predicts the number of dissatisfied people.
Predication of Thermal Comfort
• Comfort zones
• defined using isotherms parallel to ET
• ASHRAE comfort zones for summer and winter
(for typical indoor and seated person)
• proposed comfort zones
• within 5 to 16 mm Hg water vapour pressure
• for summer, 22.8 oC  SET  26.1 oC
• for winter, 20.0 oC  SET  23.9 oC
Influencing Factors
• Environmental factors:
• dry-bulb temp. (also related to humidity)
• relative humidity (or water vapour pressure)
• influences evap heat loss and skin wettedness
• usually RH between 30% and 70% is comfortable
• air velocity (increase convective heat loss)
• perferable air velocity (see Figure 4)
• mean radiation temp.
• radiation has great effect on thermal sensation
Influencing Factors
• Other factors affecting comfort:
• age
• sensation of old people and younger people
• adaptation
• people in warm climates may adapt to hot environment
• sex
• women: lower skin temp., evap loss and lower met. rate
• clothing and perferrence of temp.
What should be Estimated?
•Parameters to estimate and calculate are:
Met
Clo
Estimation of Metabolic rate
Calculation of Clo-value
Metabolic Rate
0.8 Met
8 Met
1 Met
4 Met
• Energy released by metabolism
depends on muscular activity.
• Metabolism is measured in Met
(1 Met=58.15 W/m2 body
surface).
• Body surface for normal adult is
1.7 m2.
• A sitting person in thermal
comfort will have a heat loss of
100 W.
• Average activity level for the
last hour should be used when
evaluating metabolic rate, due to
body’s heat capacity.
Met Value Table
Activity
Metabolic rates [M]
Reclining
46 W/m2
0.8 Met
Seated relaxed
58 W/m2
1.0 Met
Clock and watch repairer
65 W/m2
1.1 Met
Standing relaxed
70 W/m2
1.2 Met
Car driving
80 W/m2
1.4 Met
Standing, light activity (shopping)
93 W/m2
1.6 Met
Walking on the level, 2 km/h
110 W/m2
1.9 Met
Standing, medium activity (domestic work)
116 W/m2
2.0 Met
Washing dishes standing
145 W/m2
2.5 Met
Walking on the level, 5 km/h
200 W/m2
3.4 Met
Building industry
275 W/m2
4.7 Met
Sports - running at 15 km/h
550 W/m2
9.5 Met
Met Value Examples
Met Value Examples
Walking 3.5 km/h
2.5 MET
Jogging
8 MET
After 10 MET
Calculation of Insulation in Clothing
0.5 Clo
1.2 Clo
1.0 Clo
• 1 Clo = Insulation value of 0,155 m2 oC/W
0,15 Clo
Clo Values Table
Garment description
Iclu Clo
Iclu m2 C/W
Underwear
0.02
0.04
0.10
0.01
0.09
0.14
0.06
0.09
0.25
0.06
0.25
0.28
1.03
1.13
0.20
0.28
0.35
0.003
0.006
0.016
0.002
0.014
0.022
0.009
0.029
0.039
0.009
0.039
0.043
0.160
0.175
0.031
0.043
0.054
Underwear,
shirts
Shirts
Trousers
Insulated
coveralls
Sweaters
Pantyhose
Briefs
Pants long legs
Bra
T-shirt
Half-slip, nylon
Tube top
Short sleeves
Normal, long sleeves
Shorts
Normal trousers
Overalls
Multi-component filling
Fibre-pelt
Thin sweater
Normal sweater
Thick sweater
Clo Values Table
Garment description
Iclu Clo
Iclu m2 C/W
Jackets
0.13
0.35
0.60
0.70
0.52
0.02
0.02
0.10
0.05
0.10
0.25
0.40
0.10
0.50
0.72
0.00
0.10
0.20
0.020
0.054
0.093
0.109
0.081
0.003
0.003
0.016
0.008
0.016
0.039
0.062
0.016
0.078
0.112
0.000
0.016
0.032
Coats overtrousers
Sundries
Skirt,
dresses
Sleepwear
Chairs
Vest
Jacket
Coat
Parka
Overalls
Socks
Shoes (thin soled)
Boots
Gloves
Light skirt, 15cm above knee
Heavy skirt, knee-length
Winter dress, long sleeves
Shorts
Long pyjamas
Body sleep with feet
Wooden or metal
Fabric-covered, cushioned
Armchair
Calculation of Clo-value (Clo)
Things to consider when calculation
the CLO value
Is down better
than man made
filling?
Insulation of
wet clothing
Thermal insulation
of chairs
Acclimatisation/Adaptation!
When the air condition
system fails you can
adapt by adjusting your
CLO value
PPD (Predicted Percentage Dissatisfied)
Adjustment of Clo Value
1.2 met
1.0 Clo
Operative Temperature
0.5 Clo
What should be measured?
•Parameters to measure are:
- ta
- tr
- va
- pa
Air Temperature
Mean Radiant Temperature
Air Velocity
Humidity
Mean Radiant Temperature
Actual room
t1
Imaginary room
t4
tr
R’
R
Heat
exchange by
radiation:
R=R’
t
t3
2
• The Mean Radiant Temperature is that uniform temperature of an
imaginary black enclosure resulting in same heat loss by radiation from
the person, as the actual enclosure.
• Measuring all surface temperatures and calculation of angle factors is
time consuming. Therefore use of Mean Radiant Temperature is avoided
when possible.
Environmental Indices
• Environmental index
• express thermal comfort in a single number by
combining 2 or more comfort parameters
• operative temperature, to
• uniform temp. of an imaginary enclosure with the same
dry heat by R + C as in the actual environment
• weighted sum of tdb and tr:
hr  t r  hc  t db
to 
hr  hc
Environmental Indices
• effective temperature, ET
• temp of an environment at 50%RH that results in the
same total heat loss from the skin as for the actual
environment
• a standard set of thermal conditions representative of
typical indoor application is used to define a “standard
effective temperature (SET)”
• see Figure 5 for SET lines on psychrometric chart
Operative and Equivalent Temperature
Operative temperature
Equivalent temperature
Operative and Equivalent Temperature
Operative temperature
Equivalent temperature
Projected area factor
tr = 20 C
tr = 20 C
tr = 20 C
Operative Temperature
• The Operative temperature to integrates the effect of ta and tr
• An Operative Temperature transducer must have same heat
exchange properties as an unheated mannequin dummy.
Dry Heat Loss
• Dry Heat Loss or equivalent temperature can be measured
directly, using a heated Operative Temperature shaped
transducer.
•The Equivalent temperature teq integrates the effect of ta, tr and
va
• The Dry Heat Loss transducer is heated to the same
temperature as the surface temperature of a person’s clothing.
Comfort Temperature
1,7 CLO
2,5 MET
RH=50%
tco=6oC.
0,8 CLO
2,2 MET
RH=50%
tco=18oC.
0,5 CLO
1,2 MET
RH=50%
tco=24,5oC.
Local Thermal Discomfort
• Radiation
Asymmetry
• Draught
• Vertical Air
Temperature
Differences.
• Floor
temperature
Draught
Velocity
m/s
Time
Velocity
m/s
Time
• Draught is the most
common complaint
indoors.
• What is felt is Heat
Loss.
• Heat Loss is depending
on average Air Velocity,
Temperature and
Turbulence.
• High Turbulence is
more uncomfortable,
even with the same
Heat Loss.
Draught
• The sensation of
Draught depends on the
air temperature.
• At lower air
temperatures a higher
number will be
dissatisfied.
Mean Air Velocity
Evaluating Draught Rate
Mean Air Velocity, m/s.
15% dissatisfied
• Fluctuations in Air Velocity is
described by Turbulence
Intensity (Tu).
• Draught Rate equation is based
on studies of 150 people, and
stated in
ISO 7730.
25% dissatisfied
o
Mean Air Velocity, m/s.
Air Temperature C
o
Air Temperature C
Radiation Asymmetry
• Radiant Temperature Asymmetry is perceived
uncomfortable.
• Warm ceilings and cold walls causes greatest discomfort.
Vertical Air Temperature Difference
Dissatisfied
25 oC
Vertical Air Temperature Difference
19 oC
• Vertical Air Temperature
Difference is the difference
between Air Temperature at
ankle and neck level.
Dissatisfied
Floor Temperature
Floor Temperature
• Acceptable floor
temperatures ranging from 19
to 29 oC.
• The graph is made on the
assumption that people wear
“normal indoor footwear”.
Workplace Measurements
- 1.7 m
- 1.1 m
- 1.1 m
- 0.6 m
- 0.1 m
- 0.1 m
• Measurements of Vertical Temp. difference and Draught at ankle and neck.
• Other measurements should be performed at persons centre of gravity.
Collection of Thermal Comfort Data
Transducers
• Operative Temperature
• Air Velocity
• Radiant Temperature
Asymmetry
• Air Temperature
• Humidity
• Surface Temperature
• WBGT
• Dry Heat Loss
Air Temperature Transducer
Electrical connections
Pt100 Temperature-sensing element
Shield support
Thermal radiation shield
Surface Temperature Transducer
Pt100 Temperature-sensing element
connected to diaphragm
Spring
Electrical connections
Platinum diaphragm
Radiant Temperature Asymmetry
Transducer
Black-painted element
Side A
Gold-plated element
Shaft containing
circuit board with
preamplifier
Thermopiles
Pt100 Temperature-sensing element
Side B
Polyethylene shield
Humidity Transducer
Light-emitting diode
Cooling element
Pt100 temperature-sensing element
Conical mirror
Light-sensitive
transistor
Air Velocity Transducer
Three heated coils. For improved
frequency response, temperature and
heat loss are only measured on the
centre coil
Unheated coil of nickel wire
Plastic foam ellipsoid's
coated with white enamel
paint
Solid plastic sphere provides
protection and correction for
directional sensitivity
Shaft containing circuit
board with measuring
bridge
An Example
Comfort data logger with
comfort transducer:
• Holds 6 Comfort Transducers.
• The Mannequin is shaped as a
human body.
•Cut’s in body parts allows air
movement and radiation to
influence measurements.